1. Field of the Invention
This invention relates to batteries and a temperature-sensing apparatus for a batteries.
2. Introduction to the Invention
Secondary, i.e. rechargeable, batteries such as nickel hydride batteries and lithium ion batteries having lighter weight and larger capacity compared to conventional lead batteries have been developed and used in various applications in recent years. One application for such secondary batteries which makes use of their light weight and large capacity characteristics, is an electric vehicle which uses an electric motor as the power source, or a hybrid vehicle which combines an electric motor and an internal engine as the power source.
In order to increase the maximum voltage and current capacity available with a single secondary battery, a plurality of batteries can be connected in series in an assembly to obtain the specified voltage; these assemblies may be further connected in parallel to obtain the desired current capacity for use as a power source requiring a large driving force for applications such as the electric or hybrid vehicle.
Charging, which is required when using secondary batteries, is performed by applying the charging voltage to the assembly of secondary batteries connected in series. Since it is difficult to have uniformity in the charge and discharge characteristics of secondary batteries even with the strictest quality control, variations in the internal resistance, which changes depending on the charged state or the discharge state of the battery, are also inevitable. Therefore, when a plurality of secondary batteries having differing internal resistances are connected in series and a charging voltage is applied to the assembly of such batteries, a current passes uniformly through all the cells, causing in some cases a large rise in temperature in the secondary batteries having a large internal resistance. Such temperature rises in the secondary batteries not only shorten the life of the battery, but may also damage the casing surrounding the battery.
One way to solve such a problem is to apply the specified voltage to the two ends of each secondary battery and at the same time control the charging current, but it is difficult to provide individual charging circuits to an assembly comprising many secondary batteries.
Therefore, it becomes necessary to sense abnormal temperature rises as described above while charging secondary batteries and to stop charging the batteries which behave abnormally. One method is to sense temperature rises using a bimetal contact method. In this method, however, the contact members of the bimetallic switch may stick together when chattering caused by the on/off action at the contact occurs, so that the contact no longer functions as a switch. In addition, there can be a problem with safety.
A method utilizing a PTC (positive temperature coefficient) device has also been used to sense temperature in secondary batteries. The PTC device is in thermal contact with the object being measured so that the surface temperature of the object can be transmitted to the PTC device. The PTC device contains material which has the characteristic of exhibiting low electrical resistance at relatively low temperatures, e.g. room temperature, but exhibiting a very high resistance when exposed to high temperatures such as those which result when an abnormally large current flows through the object being measured. Previously, when PTC devices were used to sense abnormal temperature rises in secondary batteries, particularly during charging, a plurality of secondary batteries was connected in series, and devices or sensors using the PTC material were individually attached to each of the secondary batteries when being charged. In this method, however, it was not easy to attach individual sensors, and there was also the problem of complex wiring between the sensors and the temperature sensing circuit for processing data from the sensors.
We have now discovered a novel temperature-sensing apparatus which is particularly useful for monitoring the temperature of a plurality of batteries, e.g. secondary batteries installed in an electric vehicle. The present invention can be used to sense the temperature of a single secondary battery or a plurality of secondary batteries connected in series. The resulting apparatus has a low cost of manufacture and a high reliability during use, e.g. due to the presence of few welded sections in contrast to conventional sensing apparatus. In addition, the apparatus can be manufactured in various shapes. In a first aspect, the invention provides an elongate temperature-sensing apparatus which can be connected to an electrical power supply to provide a sensor for monitoring the temperature of a plurality of batteries, the apparatus comprising
(A) a positive temperature coefficient (PTC) element which
(1) is in the form of a continuous elongate tape having opposite first and second surfaces, and
(2) comprises a conductive polymer exhibiting PTC behavior;
(B) a plurality of first electrodes which
(1) are secured to the first surface of the PTC element, and
(2) are longitudinally spaced apart from each other;
(C) a plurality of second electrodes which
(1) are secured to the second surface of the PTC element, and
(2) are longitudinally spaced apart from each other,
the first and second electrodes being in a staggered and overlapping relationship, so that each electrode has (a) a central portion which does not overlap an electrode on the opposite surface and (b) two end portions which overlap respective end portions of adjacent electrodes on the opposite surface;
(D) a polymeric insulating jacket which surrounds the PTC element and the first and second electrodes,
(E) a plurality of longitudinally spaced-apart sensing components, each sensing component comprising
(1) an overlapping end portion of one of the first electrodes,
(2) an overlapping end portion of one of the second electrodes, and
(3) a portion of the PTC element, through which current runs transversely between the overlapping end portions of the electrodes when the apparatus is connected to a power supply; and
(F) a plurality of longitudinally spaced-apart connecting components, each connecting component
(1) connecting two adjacent sensing components electrically in series, and
(2) comprising
(a) a portion of the PTC element, and
(b) a central portion of one of the first electrodes or a central portion of one of the second electrodes, through which current will run longitudinally when the apparatus is connected to a power supply.
In a second aspect, the invention provides a temperature-sensing apparatus which comprises
(A) a plurality of longitudinally spaced-apart sensing components, each sensing component comprising
(1) a positive temperature coefficient (PTC) element which
(a) is in the form of a laminar sheet having opposite first and second surfaces, and
(b) comprises a conductive polymer exhibiting PTC behavior;
(2) a first electrode which is secured to the first surface of the PTC element, and
(3) a second electrode which is secured to the second surface of the PTC element, and
(B) a plurality of longitudinally spaced-apart connecting components, each connecting component lying between two adjacent sensing components and the connecting components being alternately connected
(1) to the first electrodes of the adjacent sensing components, and
(2) to the second electrodes of the adjacent sensing components;
whereby, when the apparatus is connected to a power supply, current runs transversely through the PTC elements and longitudinally through the connecting components.
In a third aspect this invention provides a process for manufacturing a temperature-sensing apparatus of the first or second aspect of the invention, said process comprising the following steps:
(A) providing a laminar element comprising
(1) a sheet having opposite first and second surfaces and composed of a conductive polymer composition exhibiting PTC behavior,
(2) a first metal sheet affixed to the first surface of the sheet, and
(3) a second metal sheet affixed to the second surface of the sheet;
(B) forming a plurality of first grooves by removing a first plurality of parallel strips from the first metal sheet;
(C) forming a plurality of second grooves by removing the a second plurality of parallel strips from the second metal sheet, wherein the first and second grooves are parallel to each other, staggered, and not overlapping;
(D) preparing a plurality of components in tape form by dividing the product resulting from step (C) along lines perpendicular to the first and second grooves; and
(E) forming a polymer insulating component around each component in tape form.
In a fourth aspect this invention provides a battery assembly comprising
(A) a plurality of batteries, and
(B) a temperature-sensing apparatus as claimed in the first aspect of the invention,
wherein each battery is provided with at least one sensing component in thermal contact therewith.